Constitutional isomers of molecular formula C₂H₇N and C₃H₉N

Introduction

All the isomers are saturated aliphatic amines - primary, secondary or tertiary (see APPENDIX).

The nomenclature of amines is very varied so I have just quoted some of the most common names used.

There are only 2 constitutional isomers of molecular formula C₂H₇N

Relative molecular mass M_r(C₂H₇N) = 45.09 

and % composition: 53.27% C,  15.68% H and 31.05% N

(based on the relative atomic masses: C = 12.01, H =1.01 and N = 14.00

Molecular formula = empirical formula = C₂H₇N

One primary amine and one secondary amine for formula C₂H₇N.

No R/S optical or E/Z geometrical stereoisomerism is possible.

You consider it functional group isomerism in terms of primary versus secondary amines.

(1) ethylamine, aminoethane or ethanamine, molecular formula C₂H₇N

condensed structural formulae, , , full displayed/graphic structural formula, , skeletal formula  , a primary aliphatic amine.

You can consider it a functional group isomer of C₂H₇N with respect to dimethylamine below.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 2 : 2 (for equivalent protons)

Index of ¹H NMR spectra organic compounds and Index of ¹³C NMR spectra organic compounds

 

(2) N-methylmethylamine, dimethylamine,   , ,

A secondary aliphatic amine.

There are not enough carbon atoms to give a tertiary amine.

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 1 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 6 (3+3) : 1 (for equivalent protons)

 

Since there are no stereoisomers, and only 2 possible constitutional isomers and therefore only 2 possible distinct isomers for molecular formula C₂H₇N

There are only 4 constitutional isomers of molecular formula C₃H₉N (3) to (6)

(3) propylamine, 1-aminopropane, propanamine or propan-1-amine

C₃H₉N, ,,  

A primary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 2 : 2 : 2 (for equivalent protons)

 

(4) 2-aminopropane, propan-2-amine , ,  

A primary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 6 (3+3) : 1 : 2 (for equivalent protons)

 

(5) N-methylethylamine, , (ethylmethylamine), N-methylethanamine

A secondary aliphatic amine.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peak areas: 3 : 2 : 3 : 1 (for equivalent protons)

 

(6) N.N-dimethylmethanamine, N,N-dimethylmethylamine, trimethylamine, ,

A tertiary aliphatic amine, very symmetrical, fewer NMR chemical shift peaks.

Here, there are now enough carbon atoms to give a tertiary amine.

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 1 ¹³C (email if disagree?)

 

Since there are no stereoisomers, there are only 4 possible constitutional isomers and therefore only 4 possible distinct isomers for molecular formula C₃H₉N

See also Amine isomers of molecular formula C₄H₁₁N

The types of isomerism exhibited by organic molecules with the molecular formulas C₂H₇N and C₃H₉N and uses

including their structural differences, physical and chemical properties, common misconceptions, and exam tips. These formulas correspond to amines and related functional groups, and are excellent examples for exploring functional group and structural isomerism.

Overview of Molecular Formulas

Types of Isomerism for the molecular formulas C₂H₇N and C₃H₉N,

1. Structural (Constitutional) Isomerism

Occurs when atoms are connected in different orders.

C₂H₇N Isomers:

C₃H₉N Isomers:

Exam Tip: Always draw full structures to distinguish between primary, secondary, and tertiary amines. Use IUPAC naming to avoid confusion.

2. Functional Group Isomerism

Same molecular formula, different functional groups.

• C₂H₇N: Ethylamine versus dimethylamine (primary versus secondary amine)

• C₃H₉N: Includes primary, secondary, tertiary amines.

Students often assume all isomers must be straight-chain. Cyclic (not here) and branched structures are valid and often overlooked.

3. Stereoisomerism

Physical Property Differences of aliphatic amines

Example: Ethylamine (CH₃CH₂NH₂) has a higher boiling point than dimethylamine due to stronger hydrogen bonding (2 protons versus 1 proton to form hydrogen bonds.

Chemical Reactivity Differences between aliphatic amines

Exam Tip: Nitrous acid reactions are a classic test for distinguishing amine types. Primary amines release nitrogen gas — a visible clue!

Common Misconceptions about isomers of C₂H₇N and C₃H₉N

• All amines behave the same: Not true — reactivity and physical properties vary significantly with structure.

• Tertiary amines can hydrogen bond: They cannot donate a H to form hydrogen bonds, only accept, H^δ+ → :N^δ-

• Cyclic amines are rare: The are often missed but valid and exam-relevant, BUT not here.

Revision Tips for questions that may involve isomers of C₂H₇N and C₃H₉N

• Use tables to compare boiling points, solubility, and reactivity.

• Practice drawing full structures and identifying lone pairs and hydrogen bonding.

• Memorize reaction outcomes with nitrous acid and acyl chlorides.

• Include cyclic (not here) and branched isomers in your revision — they often appear in higher-tier questions.

• Use functional group tests to distinguish isomers in practical scenarios.

Comparison of the mass spectra of the isomers of C₂H₇N and C₃H₉N

Here's a comprehensive comparison of the prominent diagnostic m/z ions in the mass spectra of amine isomers with molecular formulas C₂H₇N and C₃H₉N, along with common misconceptions and exam tips to help you master this topic.

Prominent Diagnostic m/z Ions from the mass spectra of the isomers of C₂H₇N and C₃H₉N

C₂H₇N Isomers (Ethylamine, Dimethylamine)

• α-Cleavage is the dominant fragmentation pathway, breaking the bond adjacent to the nitrogen atom.

• The CH₂NH₂⁺ ion (m/z = 30) is a common diagnostic fragment for primary and secondary amines.

  The mass spectrum of ethylamine

  The mass spectrum of dimethylamine

C₃H₉N Isomers (Propylamine, Isopropylamine, Trimethylamine)

• Trimethylamine shows a strong base peak at m/z = 58, not the molecular ion, due to rearrangement and loss of a methyl radical.

• Propylamine and isopropylamine show similar fragmentation but differ in relative intensities of fragment ions.

  There is quite a bit of overlap, particularly between various pairs of C₃H₉N isomers.

  The mass spectrum of propylamine (propan-1-amine, 1-aminopropane)

  The mass spectra of propan-2-amine (2-aminopropane, 2-propylamine, isopropylamine)

  The mass spectra of N-methylethylamine (N-methylethanamine, ethylmethylamine)

  The mass spectra of N,N-dimethylmethanamine (N,N-dimethylmethylamine, trimethylamine)

Common Misconceptions about the mass spectra of the isomers of C₂H₇N and C₃H₉N

• “All isomers give the same spectrum”: False. Isomers fragment differently due to structural differences.

• “The molecular ion is always the base peak”: Not true for amines. Fragment ions often dominate due to stable cation formation.

• “Mass spectra alone can't distinguish isomers”: Incorrect. Careful analysis of fragment ions and their intensities can differentiate isomers.

Exam Tips for questions that may involve the isomers of C₂H₇N and C₃H₉N

• Use the nitrogen rule: Odd molecular weight = odd number of nitrogen atoms.

• Look for α-cleavage fragments: Especially CH₂NH₂⁺ (m/z = 30) for primary/secondary amines.

• Compare base peaks: Trimethylamine often has m/z = 58 as base peak, not m/z = 59.

• Practice with NIST spectra: Real spectra help you recognize patterns and intensities.

• Draw structures and cleavage sites: Visualizing helps predict fragment ions.

Uses  and applications of amines of molecular formula C₂H₇N and C₃H₉N

A detailed overview of the uses and applications of the amine isomers with molecular formulas C₂H₇N and C₃H₉N, based on their chemical structure and industrial relevance.

C₂H₇N Isomers: Ethylamine & Dimethylamine

Ethylamine (CH₃CH₂NH₂) – Primary Amine

Applications:

• Pharmaceuticals: Used in the synthesis of drugs like antihistamines and anaesthetics.

• Agrochemicals: Intermediate in herbicide and pesticide production.

• Surfactants: Component in detergents and emulsifiers.

• Rubber industry: Vulcanization accelerator.

Dimethylamine ((CH₃)₂NH) – Secondary Amine

Applications:

• Drug synthesis: Precursor for antibiotics and other active compounds.

• Solvent: Used in organic reactions and polymer chemistry.

• Explosives: Involved in nitrosamine and stabilizer production.

• Cosmetics: Found in hair care and pH-regulating products.

C₃H₉N Isomers: Propylamine, Isopropylamine & Trimethylamine

Propylamine (CH₃CH₂CH₂NH₂) – Primary Amine

Applications:

• Chemical intermediate: Used in pharmaceutical and agrochemical synthesis.

• Surfactants: Found in cleaning agents.

• Biological research: Used in protein modification.

Isopropylamine ((CH₃)₂CHNH₂) – Primary Amine

Applications

• Herbicides: Key component in glyphosate formulations.

• Industrial chemicals: Used in corrosion inhibitors and rubber processing.

• Personal care: Present in some cosmetic products.

Trimethylamine ((CH₃)₃N) – Tertiary Amine

Applications

• Choline synthesis: Vital for brain and liver health.

• Biochemistry: Used in quaternary ammonium compound synthesis.

• Disinfectants: Active in industrial and household cleaners.

• Water treatment: Used in ion exchange resins.

Summary Table of uses of the isomers of C₂H₇N and C₃H₉N

Appendix - classification of amines

 Index of sets of isomers for a given molecular formula

Isomerism: introduction, structural isomerism - chain, positional, functional group, tautomerism

Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)

Stereoisomerism - R/S isomerism (optical isomerism) - definition - examples explained

Index of all IR, mass, 1H NMR and 13C NMR spectroscopy pages